1. Field of the Invention
The present invention relates to a receiver and an amplification-gain controlling device thereof, and more particularly, to a receiver comprising an amplification-gain controlling device for improving reception performance.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating a conventional receiver 100 comprising a automatic gain control circuit. As shown in FIG. 1, the receiver 100 includes a receiving unit 110 and a gain controlling unit 120. The receiving unit 110 includes a low noise amplifier (LNA) 111, an image-rejection mixer (IRM) 112, a low pass filter (LPF) 113, and a variable gain amplifier (VGA) 114. The gain controlling unit 120 includes a gain controller 123, the RF RSSI 121, and a comparator 122.
The LNA 111 amplifies a received signal while suppressing amplification of a noise of the received signal. The IRM 112 processes the signal by image rejection and down conversion, filters an image frequency, and removes parasitic frequencies. In other words, the IRM 112 separates an RF stage and an intermediate frequency (IF) stage and thus, secures stability of the receiving unit 110. The LPF 113 is constructed to filter a specific low band. The filtered signal is again amplified in the VGA 114. Here, the LNA 111 is not enough to amplify a weak reception signal. Therefore, a circuit construction is provided to control and amplify with an arbitrary gain using the VGA 114 for precise power control. By these all processes, an intermediate frequency signal which is amplified and has no image frequency component is outputted.
The gain controller 123 outputs a control signal for adjusting each gain of the LNA 111, the IRM 112, and the VGA 114. The RF RSSI 121 detects a strength of the signal outputted from the LPF 113. The comparator 122 of the gain controlling unit 120 sends a value for adjusting the gain to the gain controller 123 on the basis of the strength of the signal detected in the RF RSSI 121.
Because an input signal power is detected from an LPF output signal, an RF automatic gain control (AGC) loop detects a power of only a desired channel, thereby controlling a gain of a gain stage of the RF AGC loop. The RF AGC loop is comprised of the LNA 111, the IRM 112, the LPF 113, the VGA 114, the RF RSSI 121, the comparator 122, and the gain controller 123. In such case, in case where a small signal is inputted to a desired channel and a large noise signal is applied to an undesired channel together, the LNA 111 gets to be in a saturation state at the time of amplifying the small signal of the desired channel, thereby making it impossible for the receiver 100 to perform normal operation.